I’ve often been puzzled by the contradictory statistics about lifespan and smoking.

According to many reports, smoking shortens lifespan by 13.2 years for men and 14.5 years for women. (Google smoking 13.2 14.5)

Studies of smoking and life expectancy, however, tend to find that non-smokers can expect to live only about five years longer than smokers.

What’s going on?

According to standard life expectancy tables, a living 70-year-old has a remaining life expectancy of about 14 years. An 80-year-old can expect to live about 8 or 9 more years. At any age, there’s an actuarial estimate for remaining life expectancy, and it’s always a positive number.

I haven’t done the calculation, but my guess is that Americans die, on average, with an average remaining life expectancy of about 10 years.

Does this mean that "death shortens lifespan by 10 years"? No.

Consider skydivers. Death by skydiving is likely to occur at a relatively young age. Say the average age of skydivers in fatal skydiving accidents is 40. Since 40-year-olds have an average life expectancy of 39 years, is it reasonable to say that "skydiving shortens lifespan by 39 years"?

Consider (any) surgery on 75-year-olds. Some of them die from the surgery, and the life expectancy of a 75-year-old is about 11 years. Does geriatric surgery shorten lifespan by 11 years? No. Not for those who don’t die, and not for those who do, either, since on average they were probably less healthy than average to begin with.

Consider being born prematurely. The average actuarial life expectancy of a 0-year-old is about 77 years. Some premature babies die (77 years earlier than the actuarial estimate). Does being born prematurely shorten lifespan by 77 years? Not for those who live.

The often-quoted 13.2 and 14.5 year figures follow this methodology. Those numbers are the average (for men and women respectively) actuarial life expectancy estimates for people who die from a disease directly attributable to their smoking.

Since death alone, by this logic, shortens lifespan by about 10 years, death by smoking probably only knocks of a few years, not 13 or 14. And if you don’t die from smoking, who knows – maybe your death only shortens your life expectancy by 8 or 10 years.

Go figure.